Thermoelectric material, which can reversibly convert heat into electricity through the transport of internal carriers (electrons or holes), is a type of semiconductor material. If there is a temperature difference across the thermoelectric material, heat energy can be converted into electricity, which is called the Seebeck effect. As a contrast, if there is an electric field across the thermoelectric material, the electricity can be converted into heat energy, which leads to heat releasing on one side of the material while the other side will absorb heat energy. This is called the Peltier effect. Herein, thermoelectric materials can be widely used as power generation or cooling application based on the above two effects.
Generation devices made by the thermoelectric materials can be used as the power source of deep spacecraft, fieldwork, ocean lighthouse and nomadic people or directly convert industrial waste heat into electricity. Refrigeration devices made by thermoelectric material have many advantages, such as small volume and no need of chemical mediator, which can be applied as local cooling in the fields of mini freezers, computer chips, laser detectors and medical portable ultra-low temperature freezers. A wider application of thermoelectric refrigeration will also include household refrigerators, vehicle or home air conditioners. The devices made by the thermoelectric materials have many advantages such as no mechanical moving parts, no noise, no wear, simple structure and the shape or size can be designed according to the user needs.
The efficiency of a thermoelectric material is gauged by the figure of merit, zT, which is defined by:zT=(a2σT/κ)
where a, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively.
A good thermoelectric material should have high Seebeck coefficient and electrical conductivity and low thermal conductivity. High performance thermoelectric devices usually consist of high figure of merit n-type and p-type materials which should have close thermoelectric properties and crystal structure.
Nowadays, high-temperature thermoelectric materials have important applications in the fields of automotive industry, waste heat recovery and deep space satellites. The typical high-temperature thermoelectric materials are SiGe alloys, which have superior n-type thermoelectric performance with high zT of about 1.0. However, the corresponding p-type SiGe materials have relatively inferior thermoelectric performance with a low zT of about 0.5.
Recently, Half-Heusler compounds, which are consisted of earth-abundant elements, have attracted the attention of researchers in the thermoelectric field due to their excellent electrical properties. Among them, the n-type ZrNiSn-based half-Heusler compounds display high zT of about 1.0, which is comparable with the best n-type SiGe alloys. However, the p-type Half-Heusler compounds have relatively bad performance, which is a difficult problem hindering the application of Half-Heusler system as high-temperature power generation.
The raw materials of FeNbHfSb thermoelectric materials are consisted of earth-abundant and low cost elements. However, few studies can be found for this type of thermoelectric materials.